1. Field of the Invention:
This invention relates generally to bidirectional signal transceivers and, more specifically, to such transceivers providing isolated bidirectional digital communication.
2. Description of the Prior Art:
It is frequently necessary for a microprocessor to communicate with a device external to it, for example, a memory or a controller, via a bidirectional serial port of the microprocessor. The low signal levels employed make this configuration extremely sensitive to noise generated in the surrounding environment but, if the microprocessor and external device are located in proximity to each other, noise problems are substantially reduced. When, however, the application requires the microprocessor and external device be separated by an appreciable distance the noise causes substantial degradation of the signals. Accordingly, it is imperative for the effective use of such a microprocessor system that it be made immune to ambient noise.
A well-known and effective technique for overcoming noise problems, a least those of a common-mode nature, is to utilize optical coupling between the microprocessor and the external device. Of course, optical coupling can be used in any application where it is desired to electrically isolate two devices. In a unidirectional optically coupled system, an electrical signal causes emission of an optical signal representative thereof, the optical signal is detected and used to generate an electrical signal. In addition to enhancement of common mode noise rejection, the isolation afforded by optical coupling prevents problems associated with ground loops.
Depending on the application, communication between two optically coupled devices can be in a unidirectional or bidirectional mode. In unidrectional optical coupling the signal is communicated in one direction only, i.e., from device A (transmitting) to device B (receiving); there is no signal communication from device B to device A. Bidirectional communication requires both devices to receive and send signals. At times, device A will be transmitting and device B receiving, while at other times the situation will be reversed with device B transmitting and device A receiving.
Several prior art systems employ unidirectional optical coupling in a variety of applications. See, for example, U.S. Pat. No. 4,104,533 issued to Iverson; U.S. Pat. No. 4,207,557 issued to Gilkeson et al.; British Pat. No. 1,291,992 issued to Entrekin Computers Inc.; and British Pat. No. 1,330,076 issued to Landis and Gyr AG. As stated, each of these prior art techniques employs optical coupling in a single direction. The undirectional techniques employed in these prior art patents are not suitable for a bidirectional application as in the present invention, wherein one of the communicating devices has only a single terminal that alternately receives signals from and transmits signals to the other device.
Several bidirectional communication schemes are also disclosed in the prior art. See, for example, U.S. Pat. No. 3,503,061 issued to Bray et al.; U.S. Pat. No. 4,130,738 issued to Sandstedt; U.S. Pat. No. 4,119,948 issued to Ward et al.; British Pat. No. 1,378,648 issued to Computer Transmission Corporation; and U.S. patent application Ser. No. 891,997 which is assigned to the assignee of this invention and has been allowed. The common features of each of these bidirectional prior art references are: (1) Both of the communicating devices function in a transmit and a receive mode thereby providing bidirectional communication; (2) Communication between the two devices is via at least two separate optical paths wherein the first device transmits a signal to be received by the second device over a first path, and the second device transmits a signal to be received by the first device over a second path; (3) Each of the two communicating devices has separate input and output terminals for a total of four terminals.
The present invention also allows both communicating devices to operate in a transmit and a receive mode via two separate optical communication paths. However, one of the communicating devices has only a single bidirectional input/output terminal. That is, when this device is to function in the receive mode the bidirectional terminal serves as an input, and when this device is to operate in the transmit mode the bidirectional terminal serves as an output. Thus, the present invention allows bidirectional communication between devices not having matched or paired input and output terminals. These and other advantages of the present invention are discussed in detail below in the description of the preferred embodiment.